1. Field Of the Invention
The present invention relates to the field of optics and, more particularly, to spectrometric devices. Still more specifically, the invention relates to an apparatus and method for minimizing or eliminating statistical centerbursts or amplitude peaks in a multiplexed signal incorporating a plurality of periodic electromagnetic waves, such centerbursts resulting from constructive and destructive interference between the waves.
2. Description of the Prior Art
Optical spectrometric devices for measuring the distribution of light over a spectrum are well known in the art, and one class of preferred spectrophotometer devices of this character employ Hadamard multiplex techniques to elucidate the data developed during analysis. The book "Hadamard Transform Optics" by Martin Harwit, et al., published by Academic Press in 1979, provides an excellent overview of Hadamard mathematics and the degree to which common optical components are used in Hadamard spectroscopy and imaging applications; this book is incorporated by reference herein.
One type of prior art spectrophotometers use various masking devices to separate a beam of light into its spectral components for Hadamard or Fourier encoding. These prior art masking spectrophotometers are not sufficiently accurate because of signal loss experienced through mask absorption. Later, an improved spectrophotometer was developed which decreased the signal loss associated with masking spectrophotometers. Fateley U.S. Pat. No. 5,257,086, hereby incorporated by reference, discloses an optical spectrophotometer having a multi-element light source. The '086 patent provided a significant advance in the art of spectrophotometers by replacing electrically alterable masks with an electromagnetic radiation source array activated in accordance with Hadamard or Fourier transform mathematics to produce a plurality of multiplexed encodement patterns. The '086 patent discloses an electromagnetic radiation source array composed of a plurality of solid state source elements configured for allowing activation in a plurality of multiplexing encodement patterns, a detector for detecting radiation emitted from the elements and for producing signals representative thereof, and electronic controls for activating the array patterns and for producing multiplex analyses of the signals.
While the '086 patent provided a significant advance in the state of the art, problems still remain. For example, the output of spectrophotometers utilizing a source array for producing a pattern of electromagnetic waves suffer from centerbursts which degrade the performance of the spectrophotometer. Centerbursts are amplitude peaks in the multiplexed signal caused by constructive interference between the plurality of electromagnetic waves, and are statistically predictable. Centerbursts commonly occur in the output signal when the source elements are activated at the same phase or when the phases of all the source elements become momentarily equal. The amplitude of centerbursts can be very large and can saturate the detector or exceed the dynamic range of other electrical controls such as analog-to-digital converters. Moreover, when all of the source elements are energized simultaneously, the current draw on the power supply is maximized, often exceeding the output of the power supply.
One prior art solution to the centerburst problem is to use detectors with a higher detection range and power supplies with greater output capabilities. Although this straightforward solution prevents saturation of the equipment, it does so at the expense of sensitivity and economy. Detectors which can accurately measure large amplitude signals are ineffective at accurately measuring small signals. Since spectrophotometer applications require the measurement of small radiation signals, high range detectors are not suitable for these devices. Similarly, larger power supplies are a disadvantage due to their increased cost and power requirements.
Another prior art effort to solve the centerburst problem is discussed in an article by Alan G. Marshall entitled "Fourier Transforms in NMR, Optical and Mass Spectrometry." Marshall advocates a random phase variation in the source signals of different frequency, so that the source signals are initially out of phase. The phase variation is accomplished by inserting an optical element into the beam path which randomly disperses the zero-phase positions of signals and effectively "spreads out" the centerburst. Although random phase variation techniques somewhat reduce the centerburst problem, amplitude peaks still occur. More seriously, this technique introduces a barrier to the source signals and causes signal attenuation. Signal attenuation is one of the prior art problems addressed by the previously issued Fately '086 patent.
There is accordingly a need in the art for improved method and apparatus, especially in the context of spectrophotometry, which substantially minimizes or eliminates the centerburst problem without concomitant decreases in sensitively or increased signal attenuation.